By lean I don't mean needle valve lean. When a header and pipe are tuned to the correct size there is a ramming affect that causes the header and muffler or pipe to actually suck the exaust gas out of the exaust port. That is why you will get more rpm and throttler responce. It is similar to supercharging a four cycle engine. The overall volume of the header and pipe together are what is important. Imagine if you plugged up the end of a MCM and header and held it straight up and then filled it up with water. This is the volume I am talking about. You can see that if you cut a piece of the header off 1 1/2 inches you would have less volume. If you used a bigger Mouse can you would have more volume. So if you use an appropriate size Mouse can and you make sure the header is extra long or the big end pipe of the MCM is extra long you then are set what is called rich for the ramming affect. Not needle valve rich. Then when you start cutting the header or big end pipe of the MCM about a 1/4 " at a time you are gradually going LEANER.You keep cutting a little at a time until you QUIT gaining rpm. Then stop right there. Don't make another cut or you will loose the sweet spot where the header and pipe are working great.